Vehicles may be provided with a transmission designed to operate at a discrete number of fixed speed ratios. The available speed ratios may be referred to as gears or gear ratios even if there is not a direct correspondence between each ratio and a particular physical gear within the transmission. The ratios are typically numbered starting from the highest speed ratio and progressing to lower speed ratios. For example, 1st gear may have a speed ratio of 4.5, 2nd gear a speed ratio of 3.0, 3rd gear a speed ratio of 2.3, etc. At low vehicle speeds, the transmission is usually operated at a high speed ratio such that the transmission multiplies the engine torque for improved acceleration. At high vehicle speeds, the transmission is usually operated at a low speed ratio that permits an engine speed associated with quiet, fuel efficient cruising.
A particular speed ratio from the set of available speed ratios may be selected by engaging particular shift elements such as clutches, brakes, or the like. Shift elements may include actively controlled devices and passively controlled devices like one way clutches. When the currently selected speed ratio is not optimal, the transmission shifts to a different speed ratio by releasing one or more shift elements and engaging one or more shift elements. For example, if the currently selected speed ratio results in an engine speed that is higher than an optimal or efficient engine speed, the transmission may upshift to a lower speed ratio. On the other hand, if the currently selected speed ratio results in an engine speed that is less than optimal or efficient engine speed, the transmission may downshift to a higher speed ratio. During the shift, vehicle occupants may notice variations in output torque and may notice changes in the engine sound.
Modern automatic transmissions utilize controllers both to decide when to shift and to control the shift elements to accomplish the shift as smoothly as possible. Sometimes, a controller schedules a shift event in direct response to an action performed by the driver such as an abrupt change in accelerator pedal position. In these circumstances, the driver may expect that the transmission will shift and will be pleased to feel a shift. At other times, a controller schedules a shift event due to a gradual change in vehicle speed or pedal position. When a vehicle is cruising over level terrain at a steady speed, vehicle occupants expect the transmission to remain in the currently selected gear.